1. Field of the Invention
This invention relates to glass-ceramic plate surface heating units and cooktops which are provided with an electrical resistance film or foil bonded to the underside thereof for heating or cooking purposes.
2. Description of the Prior Art
In order to improve the cleanability of cooktops of domestic ranges as well as built-in counter cooktops, the standard porcelain enamel cooktop surface with separate electrical heating elements or gas burners has been replaced in certain models of appliances by high resistivity glass-ceramic plates, which are heated by either electricity or gas. Such plates are of generally milk-white, opaque, glass-ceramic or crystalline glass material sold under such trademarks as "PYROCERAM," "CER-VIT," and "HERCUVIT." This glass-ceramic material has a low thermal expansion coefficient, and it has a smooth top surface of almost ground glass finish or texture that presents a pleasing appearance and is also readily cleanable, and the continuous surface prevents the drainage of spillovers underneath the cooktop.
Most present day glass-ceramic electric surface units and cooktops use open coil radiant heaters that are separated by an air gap from the utensil-supporting glass plates as is disclosed in U.S. Pat. No. 2,913,565. Such open coil heater assemblies are less efficient thermally than standard electric cooktops with metal sheathed electrical resistance heating elements of spiral configuration which directly support the cooking utensil thereon. The glass-ceramic material has a high thermal mass thus a slow response that requires a longer time to heat up and cool down. Moreover, such open coil heaters have a poor thermal coupling between the heating element and the glass-ceramic plate and hence poor thermal efficiency. In order to transfer the heat from the open coil heater to the glass plate, the heater must operate at very high temperatures and this creates high heat losses. This tends to shorten the life of the heater, and contributes to a high heat buildup within the understructure of the surface unit or cooktop. Such open coil heaters are frequently held in place in a layer of cement within a spiral retaining groove formed in the supporting insulating block. This layer of cement encapsulates the lower portion of the open coil heater which reduces the amount of radiant energy emanating from the heating element, and further lowers its efficiency.
A more efficient open coil heating unit is taught in U.S. Pat. No. 3,612,826 which employs a sinusoidal ribbon heater that is held in place by a plurality of widely spaced staples that are fastened to the insulating support block, thus eliminating the layer of cement.
An early patent in the art of glass-ceramic plate surface heating units using film heaters in U.S. Pat. No. 3,067,315 of the present inventor and assignee. These film heaters did not create a generally uniform temperature distribution across the top surface of the plate surface heating unit.
A recently improvement patent in this art is U.S. Pat. No. 3,883,719 of the present inventor and assignee, where the film heater has a spiral pattern with a biased watts density to provide more of an even distribution of temperature across the top surface of the plate.
Another relevant patent in the art of glass-ceramic cooktops with film heaters is U.S. Pat. No. 3,813,520 where the film heater pattern includes a plurality of narrow sinuous film strips which cover a circular area of pie-shaped slices, and each strip is generally equal to each other in width and length to provide the strips with generally equal electrical resistances. If one of these film strips were to fail by being open-circuited, this heating unit would have an irregular heating pattern in the area of the failure because the film strips are bunched into areas of pie-shaped slices.
A principal object of the present invention is to provide a flat plate surface heating unit or cooktop with electrical resistance conductors of metallic film or foil formed in a pattern of a plurality of coils each having about the same watts density to provide a generally uniform temperature distribution over the top surface of the heating unit.
A further object of the present invention is to provide a glass-ceramic plate surface heating unit of the class described where each coil has about the same length and width and hence the same resistance.
A further object of the present invention is to provide a film heater of the class described with multiple loops of generally symmetrical configuration that are connected in a parallel circuit so that if one loop fails the remaining loops will continue to provide satisfactory heat distribution at a lower power output.
A still further object of the present invention is to provide a film heater of the class described by reducing the watts density of the innermost loop and increasing the watts density of the outermost loop so as to obtain a generally uniform temperature distribution over the top surface of the heating unit.